Nest predation in declining populations of common eiders Somateria mollissima: an experimental evaluation of the role of hooded crows Corvus cornix

Abstract

We evaluated the effect of the removal of hooded crows Corvus cornix on common eider Somateria mollissima nesting success using a partial Before and After Comparison of Impact (BACI) design over three years in two eider breeding colonies (Håk⊘ya and Grind⊘ya) in northern Norway. These breeding colonies had over the last decades been subject to severe declines in number of breeding birds and it was suspected that increasing nest predation by crows was contributing to the declines. Eider nesting success was monitored in both colonies during 2006‐2008. Crows were removed by live‐trapping from Håk⊘ya in 2007 and from Grind⊘ya in 2008. We monitored the number of nesting pairs of crows and general crow activity. Crow removal was generally successful in reducing the number of established territorial and visiting crows. Modelling of daily nesting success probabilities according to a logistic exposure model revealed that eider nests found at the start of the season had a much lower probability of success than nests found later on in the season. This is likely to be due in part to the increase in number of active nests during the first half of the season. The effect of crow removal appeared to differ between the two colonies. Eider nesting success on Håk⊘ya increased from 61% in the pre‐removal year 2006 to 80% during crow removal in 2007 and declined to 74% in the post‐removal year 2008. In contrast, nesting success on Grind⊘ya remained constantly low (38‐40%) during the same period. This difference between the two colonies could be explained by a difference in predation pressure, or by a higher general disturbance level on Grind⊘ya making unattended nests vulnerable to predation by a range of alternative predator species acting compensatory to the removal of crows. New investigations should be undertaken to clarify the interaction between crows and other nest predators in determining eider nesting success. Where compensation appears to occur, conditions for this process should be investigated. This will help to indicate when crow removal can be effective and which other actions can be employed to increase common eider nesting success.

Wet Grasslands as a Green Infrastructure for Ecological Sustainability: Wader Conservation in Southern Sweden as a Case Study

“…Increased numbers of avian predators may thus lead to higher nest predation rates, and subsequently cause local extirpation, or drive waders to seek alternate breeding areas (see also Norrdahl et al [89], Loman and Göransson [102]). We therefore, agree with Bell and Merton [103] and Stien et al [87] that generalist predators, such as corvids, may act to accelerate the decline in wader populations, and consequently contribute to increasing the risks of local population extinction. Hence, predator behavior and composition need to be considered at the landscape scale as applied in this study.…”

“…Additionally, Bolton et al [86] indicated that predator control can result in increased wader breeding success, but only at sites where predator densities are high. Indeed, some avian predator removal experiments have shown that the nesting success of ground nesting birds increases when predators are removed (e.g., [87,88]). However, Norrdahl et al [89] suggest that the presence of certain birds of prey, e.g., Kestrels (Falco tinnunculus), may benefit wader populations by keeping other predators away, but only if predator numbers and species are actively managed.…”

Absence from the nest due to human disturbance induces higher nest predation risk than natural recesses in Common Eiders Somateria mollissima

“…, Stien et al . , Hanssen & Erikstad ). Intrinsic factors linked to nest loss include body condition and clutch size, with individuals with poor body condition and small clutch sizes more likely to desert the nest (Hanssen et al .…”

An invasive gull displaces native waterbirds to breeding habitats more exposed to native predators

“…They often depredate the eggs and chicks of ducks and gulls. Yet, there are surprisingly few studies of the effect of corvid predation on the breeding success of waterbirds (Ewins 1991;Stien et al 2010). Corvids usually operate at the shores and edges of breeding colonies and may take up to twenty per cent of eggs in individual waterbird colonies (Montevecchi 1976(Montevecchi , 1977Burger 1984a).…”